The Collaborative International Dictionary of English v.0.48:

Martinmas \Mar"tin*mas\, n. [St. Martin + mass religious
   service.] (Eccl.)
   The feast of St. Martin, the eleventh of November; -- often
   called martlemans.
   [1913 Webster]

   Martinmas summer, a period of calm, warm weather often
      experienced about the time of Martinmas; Indian summer.
      --Percy Smith.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Martinmas
    n 1: the feast of Saint Martin; a quarter day in Scotland [syn:
         Martinmas, St Martin's Day, 11 November]